Afterglow means that a scintillation detector still emits visible light for a period of time after disappearance of an eternal excitation signal (such as a ray beam or visible light or the like) to the scintillation detector. The intensity of the afterglow approximately decreases exponentially with time.
Afterglow is generally measured by a relative intensity after the external excitation signal has disappeared for a period of time. For example, for a typical cesium iodide (thallium) (CsI (TI)) scintillation detector, the output signal of the scintillation detector is unit 1 in the presence of an external excitation signal (such as an X-ray beam). The output signal of the scintillation detector is about 5000 ppm at the moment of 10 ms after disappearance of the excitation signal, that is, the afterglow value of the scintillation detector is about 0.5% at the moment of 10 ms after disappearance of the excitation signal. Even if the scintillation detectors are of the same type, the afterglow values of the scintillation detectors produced by different manufacturers are greatly different, and different detection units have their respective afterglow values.
In an inspection system using a continuous beam-out X-ray source or an isotope radiation source, the afterglow of a scintillator of a scintillation detector is an important factor affecting the final performance index. When the substance to be detected is thick, the intensity of rays from the radiation source is attenuated to less than one thousandth, but the afterglow value may be several times of the intensity value of the attenuated rays, and the afterglow values of different detection units are different, resulting in uneven brightness of images and causing a false signal.
FIG. 1 is a scanned image formed according to a detection signal of a CsI (TI) detector. As can be seen from FIG. 1, at the same steel plate thickness, the right side is brighter than the left side, and different detectors have different afterglow values, resulting in that the image has stripes with different brightness.
FIG. 2 is a scanned image formed according to a detection signal of a cadmium tungstate detector. This scanned image is cleaner than the scanned image described in FIG. 1. Cadmium tungstate is a material with excellent scintillation performance for a scintillation detector, the afterglow time is short and the afterglow value is low.
FIG. 3 is a comparison diagram of the output relative intensity curve (thin solid line) of the CsI (TI) detector before and after disappearance of the excitation signal and the output relative intensity curve (thick solid line) of the cadmium tungstate detector. FIG. 3 represents an afterglow comparison result of a high-afterglow detector represented by a CsI (TI) detector and a low-afterglow detector represented by a cadmium tungstate detector. In FIG. 3, from −10 ms to 0 ms, an X-ray beam as an excitation signal is irradiated to the CsI (TI) detector and the cadmium tungstate detector. After the moment of 0 ms, no X-ray beam is irradiated to the CsI (TI) detector and the cadmium tungstate detector. Therefore, values of the output relative intensity curves of the CsI (TI) detector and the cadmium tungstate detector after the moment of 0 ms represent their respective afterglow values.
There are mainly two methods for solving the problem of uneven brightness of images caused by an afterglow phenomenon.
In one method, a scintillation material with low afterglow is used as the sensitive volume of the scintillation detector, but such scintillation materials are generally low in sensitivity and high in price.
In another method, the output signal of the scintillation detector is corrected for the afterglow of the scintillation detector. In this method, the afterglow of the scintillation detector needs to be measured at first, and then the effect of the afterglow is deducted from the detection result of the scintillation detector according to an algorithm routine, thereby improving the performance of the scintillation detector.
When the afterglow of the scintillation detector is measured, it is necessary to measure afterglow values of some time periods (such as 1 ms, 5 ms and 10 ms) after the X-ray (or gamma ray) is completely turned off. In order to measure the afterglow of the scintillation detector, it is necessary to provide an afterglow detection device and an afterglow detection method which are reliable, easy to use and low in cost.
In general afterglow detection methods a radiation source irradiates the scintillation detector to perform data acquisition of the scintillation detector. After the ray beam is turned off, the data acquisition of the detector is continued for a certain period of time to obtain afterglow data of the detector. The radiation source may be an electron accelerator, an isotope source, an X-ray tube, etc.
Electronic accelerators are expensive, consume a lot of power, and are difficult to popularize. The isotope source always has a gamma-ray beam, and thus has a safety problem. The X-ray tube is small in volume and easy to use, and has an energy range from tens of kilovolts to hundreds of kilovolts. The X-ray tube is generally a continuous beam-out X-ray source, and has certain advantages if it is used as a radiation source for afterglow detection.
However, when an X-ray tube is used as the radiation source for afterglow detection, after the power supply for the X-ray tube is turned off, the X-ray tube still emits X-rays, namely, residual X-rays, and the residual X-rays have an adverse effect on the afterglow detection. Both the intensity of the residual X-rays and the average energy of X-ray photons decrease with time and approximately attenuate exponentially with time. X-ray tubes of different models have different intensities of residual X-rays. Intensity of residual X-rays of a typical X-ray tube at the moment of 10 ms after the power supply is turned off can be 40% of the intensity of the X-rays when the power supply for the X-ray tube is turned on, and the average energy of the X-ray photons attenuates to about 50% of that when the power supply for the X-ray tube is turned on (X-ray tubes from different manufacturers and of different models are greatly different).
In order to reduce the adverse effect of the residual X-rays of the X-ray tube on the afterglow detection, in the related art known to the inventors, when the X-ray tube is used as the radiation source to perform the afterglow detection, it is usually necessary to configure a matched heavy metal block. When a power supply for the X-ray tube is turned off, the heavy metal block is quickly moved to a position between the X-ray tube and the to-be-detected detector to block the residual X-rays, so that the afterglow test results are protected from the effect of the residual X-rays. In order to block the residual X-rays in time, the heavy metal block needs to move at a high speed and to be stationary as soon as possible, so that the heavy metal block needs to have a large accelerated speed, and a set of relatively complicated and costly control system and mechanical system for adjusting the position of the heavy metal block is needed. At present, as the heavy metal block needs to be configured, using the X-ray tube to detect the afterglow of the scintillation detector is only used in laboratories or production workshops, but is difficult to use on product application sites.